Nebula
A nebula is an astronomical phenomena that can be present in sectors. They usually appear as strange colored shapes in the skybox of a sector.
Solar systems in a sector with a nebula will have nebula clouds in them and different background color depending on the nebula. Space dust seen when moving in space will also appear larger.
Aside from the visual effects, there is nothing interesting about nebulas.
If the visual effects of a nebula is annoying, they can be disabled in the settings.